SPECIAL ARTICLE |
a Division of Vital Statistics, National Center for Health Statistics, Centers for Disease Control and Prevention, Hyattsville, Maryland
b Department of Population and Family Health Sciences, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland
| ABSTRACT |
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The birth rate for teen mothers continued to fall, dropping 1% from 2003 to 2004 to 41.2 births per 1000 women aged 15 to 19 years, which is another record low. The teen birth rate has fallen 33% since 1991; declines were more rapid for younger teens aged 15 to 17 (43%) than for older teens aged 18 to 19 (26%). The proportion of all births to unmarried women is now slightly higher than one third. Smoking during pregnancy declined slightly from 2003 to 2004.
In 2004, 29.1% of births were delivered by cesarean delivery, up 6% since 2003 and 41% since 1996 (20.7%). The primary cesarean delivery rate has risen 41% since 1996, whereas the rate of vaginal birth after a previous cesarean delivery has fallen 67%. The use of timely prenatal care was 84.0% in both 2003 and 2004.
The percentage of preterm births rose to 12.5% in 2004 from 10.6% in 1990 and 9.4% in 1981. The percentage of low birth weight births also increased to 8.1% in 2004, up from 6.7% in 1984. Twin birth rate and triplet/+ birth rates increased by 1% and <1%, respectively, from 2002 to 2003. Multiple births accounted for 3.3% of all births in 2003.
The infant mortality rate was 7.0 per 1000 live births in 2002 compared with 6.8 in 2001. The ratio of the infant mortality rate among non-Hispanic black infants to that for non-Hispanic white infants was 2.4 in 2002, the same as in 2001. The United States continues to rank poorly in international comparisons of infant mortality.
Expectation of life at birth reached a record high of 77.6 years for all gender and race groups combined. Death rates in the United States continue to decline, with death rates decreasing for 8 of the 15 leading causes. Death rates for children
19 years of age declined for 7 of the 10 leading causes in 2003. The death rates did not increase for any cause, and rates for heart disease, influenza, and pneumonia and septicemia did not change significantly for children as a group. A large proportion of childhood deaths, however, continue to occur as a result of preventable injuries.
Key Words: birth death infant mortality low birth weight mortality multiple births cesarean delivery rate vital statistics ICD-10 revised certificates
Abbreviations: NCHSNational Center for Health Statistics IMRinfant mortality rate NMRneonatal mortality rate PNMRpostneonatal mortality rate PMRperinatal mortality rate FMRfetal mortality rate OMBOffice of Management and Budget LBWlow birth weight VBACvaginal birth(s) after previous cesarean VLBWvery low birth weight
| NOTE TO THE READER |
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Unfortunately, delays continue at both the federal and state levels, and the data reported here are both delayed and less than timely. The most recent, available mortality data reported in this article are preliminary data for 2003. The situation could be likened to that of a business that reported to its shareholders in 2006 that the most currently available information on revenues and losses was from 2003. Clearly, more timely data are needed to guide better clinical practice and inform policy decisions.
The readers of this article are the "shareholders" who provide both primary data on births and deaths and use the statistical analyses. As such, readers must rally around the need for reengineering the system and support adequate funding for all the key federal, state, and local agencies. We can't have a responsive, effective and accountable health care system in this country without more timely vital statistics.
Bernard Guyer
October 18, 2005
IN THIS ARTICLE WE provide a summary of vital statistics data through 2004. For birth data, the most current information for 2004 is based on preliminary data, whereas the more detailed analyses are based on final data for 2003. For mortality data, the most current data are 2003 preliminary data. We also include a special feature on issues relating to the rising cesarean delivery rate.
| METHODS |
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Current vital statistics patterns and recent trends through 2003 and 2004 are presented in this report by age, race, and Hispanic origin as well as other birth and death characteristics. More detailed data are available in the final birth file for 2003 than in the preliminary file for 2004, and mortality data for 2003 are limited to the preliminary file. Thus, some of the detailed analyses of birth and death patterns are not available. Data on infant deaths from the linked birth/infant death data set are final data for 2002.
Hispanic origin and race are collected as separate items in vital records. Persons of Hispanic origin may be of any race, although most births and infant deaths of Hispanic origin are to white women. Because there are often important differences in childbearing patterns between non-Hispanic white and Hispanic women, all tables that present data by race include data separately for non-Hispanic white, non-Hispanic black, and Hispanic women. Data for Native American and Asian or Pacific Islander women are not shown separately by Hispanic origin, because the vast majority of these women are not Hispanic.
The mother's marital status was reported directly on the birth certificate or through the electronic birth-registration process in all but 2 states (Michigan and New York) in 2002, 2003, and 2004. Details about the reporting of marital status in those 2 states and methods of edits and imputations that were applied to other items on the birth certificate are presented in NCHS publications.1,3,5
Cause-of-death statistics in this report are based solely on the underlying cause of death compiled in accordance with the International Classification of Diseases, 10th Revision (ICD-10).6 The underlying cause of death is defined as "(a) the disease or injury which initiated the train of morbid events leading directly to death, or (b) the circumstances of the accident or violence which produced the fatal injury."6(p30) Ranking for leading causes of death is based on number of deaths.7
Infant mortality refers to the death of an infant <1 year old. Infant mortality rates (IMRs) were computed by dividing the total number of infant deaths in each calendar year by the total number of live births in the same year.2,8 Neonatal mortality rates (NMRs) are shown for infants who died between 0 and 27 days of age, and postneonatal mortality rates (PNMRs) are shown for infants who died between 28 days and 1 year of age. Perinatal mortality rates (PMRs) include fetal deaths at
28 weeks of gestation and infant deaths at <7 days of age. Fetal mortality rates (FMRs) are shown for fetal deaths at
20 weeks of gestation. FMRs and PMRs were computed by dividing the number of fetal or perinatal deaths by the number of live births plus fetal deaths. The IMR, NMR, and PNMR are all shown per 1000 live births.
The latest infant mortality statistics by race and Hispanic origin were obtained from the 2002 period linked birth/infant death data set.8 In this data set, the death certificate was linked with the corresponding birth certificate for each infant who died in the United States in 2002. The purpose of this linkage is to use additional variables available from the birth certificate, such as birth weight, to better interpret infant mortality patterns. The numbers of infant deaths were weighted to compensate for the infant deaths for whom the matching birth certificate could not be identified.8 The weighting procedure results in the same overall IMR as that based on unlinked mortality data; however, small differences may exist because of geographic coverage differences, additional quality control, and weighting.8
Nine States (Florida [from March 1, 2004], Idaho, Kentucky, New Hampshire [from July 19, 2004], New York State [excluding New York City], Pennsylvania, South Carolina, Tennessee, and Washington) used the 2003 revision of the US Standard Certificate of Live Birth for data-year 2004.1,3 This article includes 2004 birth data for the 9 revision states and for the remaining 41 states and the District of Columbia, which reported birth data based on the 1989 revision of the US Standard Certificate of Live Birth. The prenatal-care, education, and smoking-during-pregnancy items on the 2003 revision of the US Standard Certificate of Live Birth, however, differ from those on the 1989 certificate.9,10 For the prenatal-care item, the source of the revised and unrevised data also may differ because new worksheets are used to encourage the use of the proper sources. As a result, 2004 data on these items for states that implemented the revised certificates are not compatible with those from the states that are based on the 1989 revision. Therefore, in this article, data on these items for 2004 exclude the 9 states that implemented revised certificates.1,3
Population denominators for the calculation of birth, death, and fertility rates are estimates based on the population enumerated by the US Census Bureau as of April 1, 2000. Estimates for 20002004 and the revised estimates for the intercensal period 19911999 were produced under a collaborative arrangement between the US Census Bureau and the NCHS. Reflecting the new guidelines issued in 1997 by the Office of Management and Budget (OMB), the 2000 census included an option for individuals to report >1 race as appropriate for themselves and household members.11 The 1997 OMB guidelines also provided for the reporting of Asian persons separately from Native Hawaiians or other Pacific Islanders. Under the prior OMB standards that were issued in 1977, data for Asian or Pacific Islander persons were collected as a single group.12 Birth and death certificates for most states currently collect only 1 race for mother and decedent in the same categories as specified in the 1977 OMB guidelines and do not report Asians separately from Native Hawaiians or other Pacific Islanders. Thus, birth- and death-certificate data by race (the numerators for birth and death rates) are currently incompatible with the population data that were collected in the 2000 census (the denominators for the rates).
To produce birth and death rates for 20002004 and revised intercensal rates for the 19911999 period, it was necessary to "bridge" the reported population data for multiple-race persons back to single-race categories. In addition, the 2000 census counts were modified to be consistent with the 1977 OMB race categories, that is, to report the data for Asians and Native Hawaiians or other Pacific Islanders as a combined category of Asian or Pacific Islanders.13 The procedures that were used to produce the bridged populations are described in separate publications.14,15 Rates based on bridged population data may differ from previously published rates.1,15 A number of states reported multiple-race data on birth and death certificates in 2003 and 2004. To provide uniformity and comparability of the data during the transition period before multiple-race data are available for all reporting areas, it is necessary to bridge the responses found on birth and death certificates of those who reported themselves, or were reported to have, >1 race to a single race.1 As national vital statistics data based on the 1997 OMB guidelines become available, the use of bridged populations can be discontinued.
Data for the international comparisons of births, birth rates, and IMRs were obtained from the 2002 United Nations Demographic Yearbook.16
| NATURAL INCREASE |
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| BIRTHS |
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Racial and Ethnic Composition
Fertility rates vary among race and ethnic groups (Table 2). Hispanic women had the highest fertility rate (97.7 births per 1000 women aged 15 to 44 years in preliminary 2004 data).3 Rates in 2004 were considerably lower for Asian or Pacific Islander (67.2), non-Hispanic black (66.7), Native American (58.9), and non-Hispanic white women (58.5). In 2004, 23% of all births in the United States were to Hispanic women, compared with 15% in 1990.15
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20 Years Old
Birth rates for women aged
30 have generally increased over the past 2 decades.18 From 2003 to 2004, the birth rate for women aged 30 to 34 (95.5 in 2004) increased <1%; however, the rate for women aged 35 to 39 (45.4) increased 4% and for women aged 40 to 44 (9.0) increased 3%, to its highest level in at least 30 years. After rapid increases during the 1990s, the birth rate for women aged 45 to 54 was 0.6 in 2004, an increase from 2003 (0.5).1
Unmarried Mothers
The number of births to unmarried women increased by 4% from 1415995 in 2003 to 1470152 in 2004 (preliminary data).1,3 The birth rate for unmarried women was 46.1 per 1000 unmarried women aged 15 to 44 years in 2004, up 3% from 44.9 in 2003. It remains below the peak reached in 1994 (46.9). In 2004, 35.7% of all births were to unmarried women, 3% higher than in 2003 (34.6%). This proportion has inched up slowly from 32.6% in 1994.1,19 From 2003 to 2004, the percent unmarried increased for all populations. The percents were 69.2% for non-Hispanic black women, 46.4% for Hispanic women, 24.5% for non-Hispanic white women, and 15.5% for Asian or Pacific Islander women. The percent for Native American women, which was 62.3% in 2004, did not increase significantly from 2003 to 2004.
Smoking During Pregnancy
Based on the reporting area for 2003 and 2004 of 40 states, New York City, and the District of Columbia, the percent of women who smoked during pregnancy declined slightly from 10.4% in 2003 to 10.2% in 2004 (data not shown; data for 2003 shown in Table 4 are based on 47 states and the District of Columbia).1,20 The rate has been declining and in 1989 was 19.5%. Tobacco use during pregnancy is a risk factor for a variety of adverse outcomes, including low birth weight (LBW), intrauterine growth retardation, and infant mortality, as well as negative consequences for child health.1,8,2123 In 2003, 12.4% of infants born to smokers, compared with 7.7% of infants born to nonsmokers, were of LBW, weighing <2500 g.1
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Prenatal Care
Data on prenatal care for 2004 based on unrevised certificates include 41 states, New York City, and the District of Columbia. For this reporting area, the percentage of women who began prenatal care in their first trimester of pregnancy remained essentially unchanged at 83.9% in 2004 (data not shown; 2003 data in Table 4 are for 48 states and the District of Columbia, the reporting area for this year alone). Levels of prenatal care in the first trimester rose slowly and steadily between 1990 and 2003. Efforts to reduce racial disparities in the timely receipt of prenatal care have met with some success. From 1990 to 2003, first-trimester care increased by 7% (from 83.3% to 89.0%) for non-Hispanic white women, by 24% for non-Hispanic black women (from 61.0% to 75.9%), and by 29% for Hispanic women (from 60.2% to 77.5%).
The benefits of prenatal care are difficult to measure, but timely and appropriate prenatal care may promote better birth outcomes by providing early risk assessment to manage preexisting medical conditions and by offering health behavior advice such as smoking-cessation and nutrition counseling.2426 The proportion of women who began care during the third trimester of pregnancy or had no care was 3.6% in 2004, compared with 6.1% in 1990.1,3
Cesarean Deliveries
In 2004, the cesarean delivery rate was 29.1, an increase of 6% over 2003 (Table 4), and is at the highest level reported since data have been available from birth certificates (1989).1,3,27 The cesarean delivery rate has increased steadily (41%) since 1996 (Fig 2). The rise is attributable to both an increase in the primary cesarean delivery rate (first cesarean deliveries per 100 live births to women who had no previous cesarean delivery; 20.6% in 2004) and a sharp decline in the rate of vaginal births after previous cesarean (VBAC) delivery. From 2003 to 2004, the VBAC rate fell 13% to 9.2% per 100 women with a previous cesarean delivery. It had risen 50% from 1989 to 1996 but has fallen 67% since the 1996 high of 28.3%. In 2003 and 2004, the rates of primary cesarean delivery and vaginal delivery after previous cesarean delivery may be overreported because of changes resulting from the implementation of the 2003 revision of the US Standard Certificate of Live Birth.1,3 However, for the majority of reporting areas for 20032004, there appears to be a strong upward trend in the primary rate and a sharp downturn in VBAC. Controversy continues on the risks, benefits, and long-term consequences of cesarean delivery and VBAC2831 (see "Issues Related to the Rising Cesarean Rate").
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The increase in multiple births, especially higher-order multiples, has been associated with 2 related trends: older age at childbearing and increased use of ovulation-inducing drugs and assisted reproductive technologies such as in vitro fertilization.32,33 Multiple births, regardless of how conceived, tend to be high-risk births. More than half of all twins and >90% of triplets are born preterm or with LBW; multiple births also have a greatly elevated risk of infant death.1,8 Because of the increased risk of poor outcome for these infants, the American College of Obstetricians and Gynecologists and the American Society of Reproductive Medicine issued recommendations in 1999 intended to prevent triplet+ pregnancies.34,35
Preterm Births
The percentage of births that were preterm (<37 completed weeks of gestation) increased 2% from 12.3% in 2003 to 12.5% in 2004 (Table 4). The percentage of births that were preterm rose fairly steadily since the early 1980s and was 9.4% in 1981 and 10.6% in 1990 (Fig 3). The rise in plural births, which are more likely to be born preterm, has had an influence on the rate of preterm births over the past 2 decades.1 Preterm births have higher morbidity and mortality rates when compared with term births.7 The percentage of preterm births was significantly higher only for non-Hispanic white mothers (11.5%) in 2004 (Table 4). The causes of preterm delivery are not fully understood, and until progress is made in this regard, substantial reduction in the preterm birth rate seems unlikely.1,3638
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LBW Births
LBW (<2500 g) births rose to 8.1% in 2004 from 7.9% in 2003 (Table 4; Fig 3) and has increased 21% from the 1984 low (6.7%); the current level is the highest reported since 1970.1,3 Very low birth weight (VLBW; births weighing <1500 g) births increased 1% to 1.47% in 2004. VLBW has risen from 1.15% in 1980. LBW and especially VLBW are major predictors of infant morbidity and mortality. The risk of early death for infants born at moderately LBW (15002499 g) is 6 times higher than that of heavier infants; the risk for VLBW infants is >100 times that of infants born at
2500 g.8
Between 2003 and 2004, LBW rates rose or were unchanged among each of the major racial/ethnic groups (see Table 4). Since 1990, LBW levels have climbed 29% for non-Hispanic white births (to 7.2% in 2004), 11% among Hispanic births (6.8%), and 3% among non-Hispanic black births (13.7%). Some of this increase, particularly among non-Hispanic white births, can be attributed to increases in the rate of multiple births, which tend to be born much smaller than singletons. In 2003, 58% of all plural births were born LBW.1 Among singleton births only, the increase in LBW has been more modest. Between 1990 and 2003, the LBW rate among singletons rose from 5.9% to 6.2%.1
| INFANT MORTALITY |
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Information from the linked birth/infant death data set for 2002 (the latest year available) shows important differences in IMRs according to key maternal, demographic, and health characteristics.8 Rates were higher for infants whose mothers were teenagers or
40 years of age, did not complete high school, were unmarried, began prenatal care after the first trimester of pregnancy, or smoked during pregnancy. IMRs were also higher for male infants, multiple births, and infants born preterm or LBW.
Racial differences in the IMR remain a major national concern. The relative difference in rates between the infants of non-Hispanic black and non-Hispanic white mothers expressed as a ratio was 2.4 in 2002. The Hispanic IMR was 3.4% lower than the non-Hispanic white IMR in 2002. Racial disparities in IMR present continued challenges for researchers and health care providers alike.4042
The PMR, which includes both late fetal (
28 weeks of gestation) and early neonatal (<7 days) deaths, clarifies the risk of death at late stages of pregnancy and soon after birth. For 20012002, the decline in late fetal deaths slightly outpaced the increase in early neonatal deaths, and thus the PMR remained unchanged at 6.9 deaths per 1000 live births plus fetal deaths.
The FMR (fetal deaths of at least 20 weeks of gestation) declined slightly from 6.5 per 1000 live births plus fetal deaths for 2001 to 6.4 in 2002. The late FMR (
28 weeks) declined 23% between 1990 and 2001. In contrast, the early FMR (fetal death of 2027 weeks) has not improved over this period43 or from 2001 to 2002. For 20012002, the late FMR declined another 3% from 3.3 to 3.2 per 1000.
Geographic Variation
Table 5 presents information on state variations in preterm and LBW births for 2003 (latest year for which final data are available). When examining preterm births by state, Connecticut, New Hampshire, and Vermont had the lowest rates in 2003 (9.29.7 per 1000), and Alabama, Mississippi, and Louisiana had the highest (15.617.9 per 1000). Oregon, Washington, Alaska, Minnesota, and New Hampshire had the lowest percentage of LBW births (6.06.2%), whereas Louisiana, Mississippi, South Carolina, and the District of Columbia had the highest (10.111.4%). Variations by state in preterm births and LBW reflect compositional differences by race, ethnicity, and socioeconomic status in the population in addition to other factors (eg, prenatal care and quality of care) that are associated with preterm births or LBW.
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The rank order of leading causes of death can vary by race or Hispanic origin. For example, in 2002 (as in earlier years), LBW (not congenital malformations) was the leading cause of death among non-Hispanic black and Puerto Rican infants. Deaths attributed to sudden infant death syndrome may be understated in the preliminary data because of the additional time needed to complete investigations.
International Comparisons
Table 6 shows births, birth rates, and IMRs for the United States and 26 other countries with populations >250000 and with IMRs that were less than the rate for the United States in both 2001 and 2002. The data were obtained from the United Nations Demographic Yearbook16 for 2002 for all countries except for 6 countries for which data for IMRs in 2002 were obtained from the Organisation for Economic Co-operation and Development Health Data 200544; these countries were Belgium, France, Germany, Greece, Italy, and Spain. One quarter of the countries shown in Table 6 had IMRs that were half the US rate in 2002. The US rate has remained relatively stable over the 3 years, 20002002, as have the rates for approximately half of the other countries. Only 3 countries/areas experienced a distinctive declining trend between 2000 and 2002: Hong Kong, Denmark, and Ireland.
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| DEATHS |
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Expectation of Life
The estimated expectation of life at birth for a given year represents the average number of years that a group of infants would be expected to live if, throughout their lifetime, they were to experience the age-specific death rates prevailing during the year of their birth. In 2003, the expectation of life at birth reached a new record high of 77.6 years (preliminary data), an increase of 0.3 years from the previous year.4 Life expectancy increased from the previous year by 0.4 years for black males, 0.5 years for black females, 0.3 years for white males, and 0.2 years for white females, setting record highs for the 4 groups. In 2003, life expectancy at birth was 80.5 years for white females, 76.1 years for black females, 75.4 years for white males, and 69.2 years for black males.
Causes of Death
The 15 leading causes of death in 2003 (preliminary data) accounted for >80% of all US deaths (Table 7). Between 2002 and 2003, age-adjusted death rates declined for a number of causes of death including cerebrovascular diseases by 5%, diseases of the heart by 4%, intentional self-harm (suicide) by 4%, influenza and pneumonia by 3%, pneumonitis caused by solids and liquids by 3%, chronic liver disease and cirrhosis by 2%, malignant neoplasms (cancer) by 2%, and accidents (unintentional injuries) by 2%.4 Among the 15 leading causes of death in 2003, age-adjusted death rates increased for Alzheimer's disease by 6%, hypertension by 6%, Parkinson's disease by 3%, and nephritis, nephrotic syndrome, and nephrosis (kidney disease) by 2%. Death rates for certain causes such as homicide and suicide tend to be understated in the preliminary data because of the additional time needed to complete investigations on these types of death. As a result, there will be differences between the preliminary and final data.
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Table 8 provides detailed information on the 5 leading causes of death by rank in 2003 as well as the number of deaths, the percentage accounted for by each of the leading causes, and the cause-specific death rate for each for the years 2002 and 2003. Although some causes increased or decreased in specific age groups, the overall patterns of these death rates have not changed significantly since last year.
Issues Related to the Rising Cesarean Rate
Both the total and primary cesarean rates have increased by 41% since 1996, reaching 29.1% and 20.6%, respectively, for 2004. It is of concern that some of the rise in the primary cesarean rate may be because of an increase in elective (not medically indicated) cesarean delivery. This is difficult to determine directly and has been studied by using criteria of exclusion.49
What is especially striking about the increase in the primary rate is the fact that it has been widespread for mothers of all ages, races, and ethnic groups.1,27 Once a woman has a first (primary) cesarean delivery, it is likely that subsequent pregnancies will also be cesarean deliveries. Also of concern is the 67% decrease in the rate of VBAC between 1996 and 2004 (from 28.3% to 9.2%). Rates of primary cesarean delivery and VBAC in 2003 and 2004 may be overstated because of changes resulting from the implementation of the 2003 revision of the US Standard Certificate of Live Birth.3 However, even among the states that have not implemented the 2003 revision, there has been a steep upward climb in the primary rate and a sharp reduction in VBAC.
This steep decline in the VBAC rate and increase in the primary cesarean rate may be related to more conservative practice guidelines, legal pressures, and the continuing debate regarding the harms and benefits of vaginal birth compared with cesarean delivery.29,30,31,50,51 The increase in the primary cesarean rate may also be related to reports of risks associated with VBAC.
First and repeat cesarean rates for women defined as being at low risk for a cesarean delivery by the Healthy People 2010 objectives (ie, a women with a term [at least 37 completed weeks of gestation], singleton pregnancy [not a multiple pregnancy] with vertex fetal presentation [head facing in a downward position in the birth canal]) are similar to the trends for all women.52,53
Even lower-risk first-time mothers (ie, women who meet the Healthy People 2010 criteria and, in addition, have no risk factors or complications of labor and/or delivery reported on the birth certificate) also experienced an increase in the cesarean delivery rate between 1991 and 2001.54 Based on hospital discharge data, rates of "patient-choice" cesarean delivery (ie, rates for women having a first cesarean delivery, with no labor before delivery and without certain clinical indications) have risen steadily between 2001 and 2003.55 However, if the elective or patient-choice rate is as high as 6%, as some have estimated,49,54,55 the actual number of pregnant women who had major abdominal surgery that was not medically indicated may have been as high as 41000 in 2003.
Between 1996 and 2003, total cesarean delivery rates increased at all gestational ages, with the greatest increase (
33%) for moderately preterm (3236 weeks of gestation) and term (3741 weeks of gestation) infants.1 Early (even slightly early) delivery may affect infant health.56,57
To review and evaluate the evidence concerning elective cesarean delivery, the National Institutes of Health has scheduled a State-of-the-Science Conference for March 27 to 29, 2006, on cesarean delivery by maternal request.58 The issues to be addressed by this conference (eg, short- and long-term benefits and harms to mothers and infants and future research directions to obtain evidence for making decisions regarding this subject) should impact practitioners and their patients.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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Address correspondence to Donna L. Hoyert, PhD, National Center for Health Statistics, Centers for Disease Control and Prevention, 3311 Toledo Rd, Room 7318, Hyattsville, MD 20782. E-mail: dlh7{at}cdc.gov
The authors have indicated they have no financial relationships relevant to this article to disclose.
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